196. Delete Duplicate Emails

 

Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its  smallest  Id.

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+

 

这道题让我们删除重复邮箱,那我们可以首先找出所有不重复的邮箱,然后取个反就是重复的邮箱,都删掉即可,那么我们如何找出所有不重复的邮箱呢,我们可以按照邮箱群组起来,然后用Min关键字挑出较小的,然后取补集删除即可:

 

解法一:

DELETE FROM Person WHERE Id NOT IN
(SELECT Id FROM (SELECT MIN(Id) Id FROM Person GROUP BY Email) p);

 

我们也可以使用内交让两个表以邮箱关联起来,然后把相同邮箱且Id大的删除掉,参见代码如下:

 

解法二:

DELETE p2 FROM Person p1 JOIN Person p2 
ON p2.Email = p1.Email WHERE p2.Id > p1.Id;

 

我们也可以不用Join,而直接用where将两表关联起来也行:

 

解法三:

DELETE p2 FROM Person p1, Person p2
WHERE p1.Email = p2.Email AND p2.Id > p1.Id;

 

类似题目:

Duplicate Emails

 

参考资料:

https://leetcode.com/discuss/61176/simple-solution-using-a-self-join

https://leetcode.com/discuss/48403/my-answer-delete-duplicate-emails-with-double-nested-query

 

LeetCode All in One 题目讲解汇总(持续更新中…)


转载请注明来源于 Grandyang 的博客 (grandyang.com),欢迎对文章中的引用来源进行考证,欢迎指出任何有错误或不够清晰的表达。可以在下面评论区评论,也可以邮件至 grandyang@qq.com

💰


微信打赏


Venmo 打赏

×

Help us with donation